Polishing composition and polishing method using the same

ABSTRACT

A polishing composition contains sodium hypochlorite, colloidal silica and water. The effective chlorine concentration in the polishing composition is 0.5 to 2.5%, and the content of colloidal silica in the polishing composition is 1 to 13% by weight. The polishing composition can be preferably used in polishing germanium or silicon-germanium single crystal.

BACKGROUND OF THE INVENTION

The present invention relates to a polishing composition for use in polishing germanium or silicon-germanium single crystal, and a method for polishing single crystal surfaces.

Although germanium single crystal has the problems of a narrow band gap and being unstable at high temperatures, since its carrier (electron) mobility is two to three times larger than that of silicon, it is attracting attention as a material for high-speed semiconductor devices (see, for example, K. Ismail et al., Identification of a Mobility-Limiting Scattering Mechanism in Modulation-Doped Si/SiGe Heterostructures, Physical Review Letters, USA, 1994, Vol. 73, pp. 3447-3450). In addition, since silicon-germanium single crystal also has high electron mobility, it is similarly attracting attention as a material for high-speed semiconductor devices, high-frequency devices and so on.

During the production of germanium or silicon-germanium semiconductor devices, the surface of a germanium single crystal substrate or the surface of a germanium or silicon-germanium single crystal film formed by epitaxial growth on a silicon single crystal substrate is polished to a mirrored surface. The purpose of this polishing is to remove scratches formed in the germanium single crystal substrate during lapping, and to remove hatching formed in the surface due to lattice mismatch between a silicon single crystal substrate and silicon-germanium single crystal film.

Conventionally, an etching agent in the form of a sodium hypochlorite solution was used together with a polishing cloth or polishing pad when polishing germanium or silicon-germanium single crystal as disclosed in U.S. Pat. No. 3,342,652. However, according to this method, it was difficult to obtain a mirrored surface due to the appearance of an orange peel pattern or surface roughness in the surface after polishing.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a polishing composition that can be suitably used in polishing germanium or silicon-germanium single crystal, and a polishing method that uses such a polishing composition.

In order to achieve the above object, one aspect of the present invention provides a polishing composition for use in polishing germanium or silicon-germanium single crystal. The polishing composition contains sodium hypochlorite, colloidal silica and water. The effective chlorine concentration in the polishing composition is 0.5 to 2.5%. The content of colloidal silica in the polishing composition is 1 to 13% by weight.

Another aspect of the present invention provides a method for polishing germanium or silicon-germanium single crystal. The method includes preparing the above polishing composition, and using the prepared polishing composition to polish germanium or silicon-germanium single crystal.

Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawing, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawing in which:

FIG. 1 is a schematic drawing of a polishing apparatus used in the examples and comparative examples of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention will now be described.

A polishing composition according to the present embodiment is used in polishing a germanium single crystal substrate, or in polishing a germanium or silicon-germanium single crystal film provided on a silicon single crystal substrate. The polishing composition is produced by mixing predetermined amounts of sodium hypochlorite and colloidal silica in water, and contains sodium hypochlorite, colloidal silica and water.

Sodium hypochlorite has the effect of chemically etching germanium or silicon-germanium single crystal to improve the removal rate of germanium or silicon-germanium single crystal. In addition to sodium hypochlorite, basic substances such as potassium hydroxide, ammonium hydroxide and ethylenediamine, as well as an oxidizing agent other than sodium hypochlorite, such as hydrogen peroxide, periodates, orthoperiodic acid and permanganates have the effect of chemically etching germanium or silicon-germanium single crystal. However, the etching ability of basic substances is weaker than that of sodium hypochlorite, and basic substances do not allow the removal rate to significantly be improved. In addition, oxidizing agents other than sodium hypochlorite include those like hydrogen peroxide that have a weak etching ability. Although there are some oxidizing agents other than sodium hypochlorite, such as periodates and orthoperiodic acid that have etching ability equivalent to that of sodium hypochlorite, in the case of using these instead of sodium hypochlorite, corrosion (defects) easily occurs in the polished surface. Moreover, hydrogen peroxide, orthoperiodic acid and permanganates easily corrode polishing pads. In contrast, when sodium hypochlorite is used, irregularities are hardly left in the polished surface and polishing pads rarely undergo corrosion while sodium hypochlorite has high etching ability.

Colloidal silica has the effect of mechanically polishing germanium or silicon-germanium single crystal to improve the removal rate of germanium or silicon-germanium single crystal. In addition to colloidal silica, zirconia also has the effect of mechanically polishing germanium or silicon-germanium single crystal. However, in the case of using zirconia instead of colloidal silica, the quality of the polished surface decreases. In contrast, colloidal silica has the polishing action equivalent to that of zirconia, and, when colloidal silica is used, the quality of the polished surface improves.

Colloidal silica to be contained in the polishing composition is preferably as free of impurities as possible. In particular, if impurities such as sodium, aluminum, titanium, iron, nickel, copper, their hydroxides or their oxides are contained in the polishing composition, those impurities adhere to the surface to be polished and become dispersed in the single crystal in subsequent heat treatment resulting in the risk of imparting detrimental effects on the electrical characteristics of the single crystal. Thus, colloidal silica to be contained in the polishing composition is preferably high-purity colloidal silica. More specifically, in the case of preparing an aqueous dispersion of colloidal silica using colloidal silica to be contained in the polishing composition so that the content of colloidal silica is 20% by mass, the total content of sodium, aluminum, titanium, iron, nickel and copper in the aqueous dispersion (amount of impurities) is preferably 100 ppm or less, more preferably 1 ppm or less, and most preferably 0.3 ppm or less. In contrast to ordinary colloidal silica being synthesized by hydrolysis of sodium silicate, high-purity colloidal silica is synthesized by hydrolysis of alkoxysilane.

The effective chlorine concentration in the polishing composition is 0.5 to 2.5%. If the effective chlorine concentration is less than 0.5%, a high removal rate is unable to be obtained. If the effective chlorine concentration exceeds 2.5%, the quality of the polished surface decreases due to the appearance of an orange peel pattern and so forth. The effective chlorine concentration is measured by, for example, oxidation-reduction titration.

The content of colloidal silica in the polishing composition is 1 to 13% by weight. If the content of colloidal silica is less than 1% by weight, a high removal rate is unable to be obtained. Even if colloidal silica is contained in excess of 13% by weight, there is no significant improvement in the removal rate, thus making this impractical. In the case of having set the content of colloidal silica in the polishing composition to 3 to 7% by weight, the occurrence of orange peel pattern is further inhibited. Thus, the content of colloidal silica in the polishing composition is preferably 3 to 7% by weight.

The aforementioned embodiment may be modified in the manner described below.

The polishing composition according to the aforementioned embodiment may be used in a CMP process when producing LSIs comprising FETs using germanium or silicon-germanium single crystal.

Acids such as sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, acetic acid or citric acid, or bases such as potassium hydroxide, sodium hydroxide or ammonium hydroxide may be added as a pH adjusting agent to the polishing composition according to the aforementioned embodiment as necessary.

Water-soluble polymer compounds such as hydroxymethyl cellulose, chloromethyl cellulose, polyacrylate, and polyvinyl acetate, nonionic surfactants such as nonylphenyl ether, polypropylene glycol, polyethylene glycol, and monoalkyl or dialkyl ethers of polyethylene glycol or polypropylene glycol, or anionic surfactants such as sodium alkylbenzene sulfonate may be added to the polishing composition according to the aforementioned embodiment for the purpose of viscosity, adjusting friction and so forth.

The following provides an explanation of examples and comparative examples of the present invention.

Test 1

Sodium hypochlorite and high-purity colloidal silica (amount of impurities: 0.04 ppm, average primary particle size: 0.035 μm, average secondary particle size: 0.07 μm) were mixed as appropriate into deionized water to prepare polishing compositions. The effective chlorine concentration and colloidal silica content in each polishing composition are as shown in Table 1. The average primary particle size of colloidal silica was calculated from the specific surface area as measured with the Flow Sorb II 2300 manufactured by Micromeritics Corp. The average secondary particle size of colloidal silica was determined from the particle size distribution as measured with the ELS-8000 manufactured by Otsuka Electronics Co., Ltd.

When a germanium single crystal substrate was polished using each of the resulting polishing compositions under the polishing conditions shown in Table 2, the removal rates shown in Table 1 were obtained. As shown in Table 1, in the case of polishing using a polishing composition containing 1% by weight or more of colloidal silica, the removal rate was larger than in the case of using a polishing composition not containing colloidal silica. In the case where the colloidal silica content was less than 3% by weight or greater than 7% by weight, there was some occurrence of a visually perceivable orange peel pattern. In contrast, in the case of the colloidal silica content of 3 to 7% by weight, there was no occurrence of a visually perceivable orange peel pattern, and a satisfactory polished surface was obtained. TABLE 1 Effective chlorine SiO₂ content Removal rate concentration (%) (wt %) (μm/hr) 1.5 0 14.0 1.5 1 18.7 1.5 2 21.3 1.5 3 20.4 1.5 5 21.4 1.5 7 22.3 1.5 10 20.7 1.5 13 21.3

TABLE 2 Polishing apparatus Single-sided polisher having a surface plate diameter of 300 mm manufactured by Engis Japan Ltd. Polishing pad Politex Supreme High Nap manufactured by Nitta Haas Inc. Object to be Three germanium single crystal polished substrates having a diameter of 50 mm (P type, plane direction (100)), surface roughness Ra = 1.0 nm Surface plate 60 rpm rotation speed Polishing pressure 98.1 hPa Polishing time 15 minutes Polishing composition 1 mL/minute feed rate

FIG. 1 is a schematic drawing of the polishing apparatus used for polishing under the conditions shown in Table 2. As shown in FIG. 1, the polishing apparatus is provided with a rotating surface plate 11, a wafer holder 12, and a polishing composition feeder 13. A polishing pad 14 is adhered to the upper surface of the rotating surface plate 11. A ceramic plate 15 is retained on the lower surface of the wafer holder 12.

When objects 16 are polished using the polishing apparatus, the objects 16 are first fixed to the lower surface of ceramic plate 15 using a solid wax. Subsequently, a polishing composition is started to be fed to the polishing pad 14 at a predetermined feed rate (polishing composition feed rate) from the polishing composition feeder 13. At the same time, the rotating surface plate 11 is rotated at a predetermined rotation speed (surface plate rotation speed) in the direction of arrow A as shown in FIG. 1 while pressing the wafer holder 12 against the rotating surface plate 11 so that the objects 16 make contact with the polishing pad 14 at a predetermined pressure (polishing pressure). Since the ceramic plate 15 rotates in the direction of arrow B as shown in FIG. 1 accompanying the rotation of the rotating surface plate 11, the objects 16 are polished by the rotating polishing pad 14 while the objects 16 themselves rotate.

<Test 2>

Polishing compositions having different effective chlorine concentrations and colloidal silica contents were prepared as shown in Table 3. The removal rates measured when a germanium single crystal substrate was polished under the polishing conditions shown in Table 2 using each of the resulting polishing compositions are shown in Table 3. It was determined from these results that the removal rate increased as the effective chlorine concentration and colloidal silica content increased. In addition, the removal rate was found to increase nearly proportionate to the effective chlorine concentration. The removal rate was less than 1.0 μm/hr when the effective chlorine concentration was 0.0%. TABLE 3 SiO₂ content Effective chlorine concentration (%) (wt %) 0.0 0.5 1.0 1.5 2.0 2.5 0 <1.0 5.3 — 14.0 — 20.0 2 <1.0 6.7 12.0 21.3 — 31.0 5 <1.0 6.0 12.0 21.4 24.4 30.4 7 <1.0 7.3 12.0 21.3 — 30.0 (The units for the removal rates in Table 3 are μm/hr.)

Test 3

Sodium hypochlorite and colloidal silica (amount of impurities: less than 0.3 ppm, primary particle size: 0.035 μm, secondary particle size: 0.07 μm) were mixed as appropriate into deionized water to prepare polishing compositions. The effective chlorine concentration and colloidal silica content in each polishing composition are as shown in Table 4. The removal rates measured when a germanium single crystal substrate was polished under the polishing conditions shown in Table 2 using each of the resulting polishing compositions are shown in Table 4. As shown in Table 4, the removal rates were greater in the case of an effective chlorine concentration of 0.5% or more as compared with an effective chlorine concentration of less than 0.5%. In addition, a visually perceivable orange peel pattern was formed and surface roughness able to be observed with a Nomarski Microscope occurred when the effective chlorine concentration exceeded 2.5%. In contrast, there was no occurrence of a visually perceivable orange peel pattern or surface roughness able to be observed with a Nomarski Microscope when the effective chlorine concentration was 2.5% or less. TABLE 4 Effective chlorine Si0₂ content Removal rate concentration (%) (wt %) (μm/hr) 0.0 2 <1.0 0.5 2 6.7 1.0 2 12.0 1.5 2 21.3 2.5 2 31.0 4.0 2 31.7 5.0 2 25.3

Test 4

Polishing compositions containing 2% by weight of any of colloidal silica, zirconia, fumed silica or alumina (γ-alumina) as abrasives and containing sodium hypochlorite so as to have the effective chlorine concentration of 1.5% were prepared. A germanium single crystal substrate was polished under the polishing conditions shown in Table 2 using each of the resulting polishing compositions. The removal rates measured at that time and the surface roughness Ra of the substrates after polishing as measured under the measurement conditions shown in Table 6 are shown in Table 5. As shown in Table 5, although the polishing composition that contains colloidal silica as abrasives demonstrated a somewhat low value for the removal rate, an extremely low value was demonstrated for surface roughness Ra. On the other hand, although the polishing composition that contains zirconia as abrasives demonstrated the highest removal rate, the value for surface roughness Ra was large. The polishing composition that contains fumed silica or alumina as abrasives demonstrated large values for surface roughness Ra. On the basis of these results, colloidal silica was determined to be preferable as compared with zirconia, fumed silica and alumina.

Colloidal silica used here had an average primary particle size of 0.035 μm and an average secondary particle size of 0.07 μm. Fumed silica used here had an average primary particle size of 0.03 μm and an average secondary particle size of 0.1 μm. Zirconia used here had an average primary particle size of 0.05 μm and an average secondary particle size of 0.2 μm. Alumina used here had an average primary particle size of 0.05 μm and an average secondary particle size of 0.2 μm. The average primary particle size of the colloidal silica, fumed silica, zirconia and alumina were calculated from specific surface area as measured with the Flow Sorb II 2300 manufactured by Micromeritics Corp. The average secondary particle size of the colloidal silica and fumed silica were determined from the particle size distribution as measured with the ELS-8000 manufactured by Otsuka Electronics Co., Ltd. The average secondary particle size of the zirconia and alumina were measured with the Microtrac (Registered) 9200 FRA manufactured by Microtrac Corp. TABLE 5 Removal rate Surface roughness Abrasives (μm/hr) Ra (nm) Colloidal silica 21.3 0.8 Fumed silica 32.0 4.1 Zirconia 40.0 2.6 Alumina 34.7 6.0

TABLE 6 Measuring instrument MicroXAM manufactured by Phase Shift Corp. Lens Objective lens of 10× magnification factor and intermediate lens of 10× magnification factor Filter 80 μm low pass filter Measuring Range 163 μm × 123 μm

Test 5

Polishing compositions were prepared containing 2% by weight of colloidal silica along with any of sodium hypochlorite, hydrogen peroxide and orthoperiodic acid as an oxidizing agent. The removal rates measured when a germanium single crystal substrate was polished under the polishing conditions shown in Table 2 using each of the resulting polishing compositions are shown in Table 7. As shown in Table 7, the removal rate was low in the case of using hydrogen peroxide for the oxidizing agent. Although the removal rate in the case of using orthoperiodic acid for the oxidizing agent was larger than in the case of using sodium hypochlorite for the oxidizing agent, surface roughness was observed when the substrate was observed with a Nomarski Microscope after polishing. In addition, corrosion of the polishing pad was found to occur in a different test in the case of the polishing composition containing a high concentration (e.g., 20% or more) of orthoperiodic acid. TABLE 7 Content Removal rate Oxidizing agent (wt %) (μm/hr) Sodium hypochlorite 1.0 12.0 Hydrogen peroxide 6.0 5.3 Orthoperiodic acid 6.0 15.3 (The content of sodium hypochlorite in Table 7 shows the effective chlorine concentration.) 

1. A polishing composition for use in polishing germanium or silicon-germanium single crystal, the polishing composition comprising sodium hypochlorite, colloidal silica and water, wherein the effective chlorine concentration in the polishing composition is 0.5 to 2.5%, and the content of colloidal silica in the polishing composition is 1 to 13% by weight.
 2. The polishing composition according to claim 1, wherein the content of colloidal silica in the polishing composition is 3 to 7% by weight.
 3. The polishing composition according to claim 1, wherein, when an aqueous dispersion of colloidal silica is prepared using colloidal silica to be contained in the polishing composition so that the content of colloidal silica is 20% by mass, the total content of sodium, aluminum, titanium, iron, nickel and copper in the aqueous dispersion is 100 ppm or less.
 4. The polishing composition according to claim 1, wherein colloidal silica in the polishing composition is synthesized by hydrolysis of alkoxysilane.
 5. A method for polishing germanium or silicon-germanium single crystal, the method comprising: preparing a polishing composition including sodium hypochlorite, colloidal silica and water, wherein the effective chlorine concentration in the polishing composition is 0.5 to 2.5%, and the content of colloidal silica in the polishing composition is 1 to 13% by weight; and using the prepared polishing composition to polish germanium or silicon-germanium single crystal. 